Just to be a bit more precise:
It's called pretermited heir and it's not allowed.
That's true if the will was written BEFORE he knew the OP was an heir. If the will was written AFTER he knew the OP was an heir, but still left her out, then she is an omitted heir.
The only way to legally disinherit a child is to specifically mention that child in the will.
Not entirely true. In Colorado, if the child was known to the testator when the will was written, but the child was not included in the will -- in others words, an omitted rather than a pretermitted child -- then the omitted child takes nothing, unless it can be proven that the testator thought the child was dead when the will was written. See Col. Rev. St. 15-11-302 (note that Colorado, though, uses "omitted child" to describe a pretermitted child in the statute).
Some states do require that a child be specifically mentioned in a will to be disinherited, and its probably always a good idea to specifically disinherit a child if that what you want to do (simply so that it is clear, and to reduce the probability of a will contest). But in many, if not most states, a will written when the testator knew of the child that leaves out the child is sufficient to raise the presumption (which may be overcome in very limited circumstances, like when the testator thinks the child is dead when the will was written) that the testator meant to disinherit the child.
That said, BB's advice is sound -- if the will was written before he knew about you, just sit back and wait, and then you can take your share from the shares of the other 4 daughters. If he does rewrite the will and leaves you out, you are going to have a more difficult time, because then you would have to prove that he lacked the capacity to write a new will. But if the will was written before he knew about you, take BB's advice and lay low.
